Spacewalkers Tracy Caldwell Dyson (l.) and Doug Wheelock install the ammonia coolant pump module on the International Space Station in this image from NASA TV August 16.

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Two spacewalking astronauts ventured outside the International Space Station Monday on another big repair job to install a vital new pump and restore their ailing cooling system back to full strength for the first time in two weeks.

The space station's cooling system has been running at half strength since July 31, when an electrical short shut down one of two pumps that move liquid ammonia through the system. The failure forced astronauts to turn off some experiments and systems, as well as leave others without backups, in order to prevent the station from overheating. [Graphic: Space Station's Cooling System Problem Explained]

Space station managers hope Wheelock and Caldwell Dyson will be able to completely hook up the new pump during Monday's spacewalk. The astronauts removed the faulty ammonia pump over two previous spacewalks (on Aug. 7 and Aug. 11) and are expected to install to perform a fourth spacewalk to move the disabled pump to its final storage location.

Mike Suffredini, NASA's space station program manager, said Monday's spacewalk – or extravehicular activity in NASA parlance – would go as long as possible to complete as much work as possible.

"The EVA will be as long as the consumables will allow," Suffredini said last week. "We'll take all the time we can get because we won't be done."

There are two main cooling system loops – Loop A and Loop B. The failed pump is in Loop A, while the other cooling loop remains operational. It was delivered to the space station in 2002, but wasn't activated until 2006.

NASA station managers have said the ammonia pump failure has been one of the most challenging repairs for the International Space Station ever attempted. The cooling system is so critical to station operations that a pump repair is one of 14 major failures for which NASA engineers have prepared emergency plans for in advance, they added.

There are four spare ammonia pumps on the space station, one of which will be used for this repair. Each pump weighs 780 pounds (353 kg) and is 5 1/2 feet long (1.6 meters) by 4 feet wide (1.2 meters).

The $100 billion International Space Station is currently home to six astronauts; three Americans and three Russians. Spare parts are key concern since the station is slated to keep flying through at least 2020. NASA plans to fly two more shuttle missions (in November and February 2011) before retiring its three remaining space shuttles for good. A potential third shuttle flight, which would launch next summer if approved, is also under discussion in Congress.

Once the space shuttle fleet is retired, NASA will rely on spacecraft from Russian to launch crews and cargo to the space station until American-built commercial spaceships or new government vehicles become available. Unmanned cargo ships built by the Japanese and European space agencies are also expected to keep the station stocked with supplies.